Turbochargers are a type of forced induction system. They deliver air, at greater density than would be possible in the normally aspirated configuration, to an engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. This can enable the use of a smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, thus reducing the mass and aerodynamic frontal area of the vehicle.
The rotating assembly of a turbocharger rotates at exceptionally high speed in the order of 200,000 RPM for a small rotor and 80,000 RPM for a large rotor. Typically, the rotating assembly of a turbocharger is supported by a class of bearing known as an internally pressurized or hydrodynamic bearing. In this class of bearing, the rotation of the shaft, relative to the bore in which it resides, drags the lubricant in the direction of rotation, causing the generation of a wedge of oil between the relatively low speed element and the relatively high speed element. The squeeze film nature of the system causes an asymmetric force to be exerted on the shaft. The magnitude of the force applied to the bearing by the unbalance is proportional to the shaft speed squared.
FIG. 1 shows a cross-sectional view of a typical floating bearing turbocharger (10). The turbocharger (10) includes a turbine stage (12) and a compressor stage (14). Turbochargers use the exhaust flow from the engine exhaust manifold to drive a turbine wheel (16), which is located in a turbine housing (not shown). Once the exhaust gas has passed through the turbine wheel (16) and the turbine wheel (16) has extracted energy from the exhaust gas, the spent exhaust gas exits the turbine housing through an exducer and is ducted to the vehicle downpipe and usually to after-treatment devices such as catalytic converters, particulate traps, and NOx traps. The energy extracted by the turbine wheel (16) is translated to a rotational motion, which then drives a compressor wheel (18). The compressor wheel (18) draws air into the turbocharger (10), compresses this air, and delivers it to the intake side of the engine. The rotating assembly consists of the following major components: turbine wheel (16); a shaft (20) upon which the turbine wheel (16) is mounted; a compressor wheel (18) also mounted on the shaft (20); an oil flinger (22); and thrust components. The shaft (20) has an associated axis of rotation (21).
The shaft (20) rotates on a hydrodynamic bearing system which is fed oil, typically supplied by the engine. The bearing system can be provided in a bearing housing (23). The oil is delivered via an oil feed port (24) to feed both journal bearings (26) and thrust bearings (28). The same system is typically used on each of the two journal bearings in a turbocharger. The function of the journal bearing is to control, maintain, and damp the radial oscillations of the rotating assembly. A separate thrust bearing (28) controls and maintains the axial position of the rotating assembly relative to the aerodynamic features in the turbine housing and compressor housing (not shown). The thrust loads are typically carried by ramped hydrodynamic bearings working in conjunction with complementary axially-facing rotating surfaces of a pair of thrust washers (26). In some turbochargers, one thrust washer is part of the oil flinger (22), and the other becomes part of the complementary assembly. In other turbochargers, the thrust washer is a single part with two opposing faces fabricated onto a cylindrical part, in the shape of a bobbin, and the bottom segment of the thrust bearing bore remains open, like a horseshoe, to accept the thrust washer. Once used, the oil drains to the bearing housing (23) and exits through an oil drain (32) fluidly connected to the engine crankcase.
With a typical floating journal bearing, there are two hydrodynamic films in action. As depicted in FIG. 2, one hydrodynamic film operates between the surface of the static inner bore (34) of the bearing housing (23) and the rotating outer peripheral surface (36) of the journal bearing (26). The other hydrodynamic film operates between the rotating inner peripheral surface (38) of the journal bearing (26) and the faster-moving outer peripheral surface (40) of the shaft (20). While the bearing housing (23) is static (relative to the shaft (20)), the journal bearing (26) rotates at approximately 10% to 30% of the speed of the shaft (20), depending upon the torque balance between the outer oil film torque and the inner oil film torque. The journal bearings (26) are typically held in place by the use of retaining rings (42) which locate in grooves in the bearing housing (23).
A taper-land journal bearing system typically has a “non-rotating bearing”, which has only one rotating oil film system, and that is the system between the outside surface of the rotating shaft and the inside surface of the bore in the rotationally static bearing. In this bearing design, there typically still exists a non-rotating oil film between the outside surface of the bearing and the inside surface (23) of the bearing housing. The latter is used as a damping mechanism only, producing no sub-synchronous response. A typical taper land bearing is prevented from rotation while being able to float and be damped by the outer oil film through a non-rotation device. In addition to the ability of the bearing system to withstand unbalance loads, the removal of one of the rotating fluid films (e.g., the fluid film between the outer peripheral surface (36) of the journal bearing (26) and the inner peripheral surface of the bearing housing bore (34)) reduces the path of the sub-synchronous vibration transmittance, which removes the opportunity for the vibration to be transmitted through the bearing housing as an objectionable noise. The inner oil film system of a taper land design uses a pair of axially defined three pocket taper-land features external to the annulus formed by the surface of the basic inside diameter of the non-rotating bearing and the outer peripheral surface (40) of the rotating shaft (20) to artificially create a stabilizing balance load on the rotating shaft of the rotating assembly.
The requirements of the functions of the bearings determine in part the axial length of the turbocharger in that the bearings are placed to minimize the overhung masses of the wheels. The thrust bearing is typically outboard of the compressor-end journal bearing, thus adding more to the axial length of the turbocharger. The axial span of the bearings becomes a critical element in the axial length of the turbocharger and thus a critical element in the fitting of a turbocharger, or more commonly multiple turbochargers, into the engine bay of a vehicle.
Engine compartments of modern vehicles are always compact to reduce the frontal area of the vehicle, and this presents difficulties in the fitting of turbochargers to the various interfaces on the vehicle. With the endeavors of the vehicle manufacturers to improve the aerodynamics of the vehicle, coupled with the drive for better fuel efficiency of the engine, while meeting ever more stringent emissions, the space allocated for the turbocharger is becoming more problematic. With the advent of an ever increasing acceptance of two stage turbochargers, vehicle manufacturers are now trying to fit two turbochargers into the space allocated for a single turbocharger so the size of the turbocharger is becoming more important.
Thus, there is a need for a system for reducing the axial length of a turbocharger.